A low power wireless power receiver, for example, rated at 5 Watts (W), when receiving power from a higher power wireless power transmitter, for example, rated at 65 W needs to be protected against excessive power potentially being transmitted from the wireless power transmitter. Conventional wireless power receivers typically have a clamping mechanism to guard against harmful voltage and current conditions which occur when excessive power is received. These clamping mechanisms divert the excess power away from the main circuitry of the wireless power receiver thereby protecting the main circuitry of the wireless power receiver. However, the clamping mechanisms can drain away only up to a certain amount of excess power. This power limit of the clamping mechanism is set at the time of designing the clamping mechanism. When the excess power during the actual operation exceeds this power limit, the clamping mechanism will be damaged and will not be able to protect the main circuitry of the wireless power receiver any further. In that situation, the main circuitry of the wireless power receiver will also suffer permanent damage and the wireless power receiver will become non-functional.
As the lower rated power wireless power receivers could easily be exposed to unsafe power levels while being powered from a higher rated power wireless power transmitter, low power wireless power receivers are very susceptible to damages when used with such wireless power transmitters. Hence, there is a need for a wireless power system that constrains the higher rated power wireless power transmitter to transmit a known level of maximum power which does not exceed the safe limits of the wireless power receiver.
There are several wireless power receivers in the market today such as those built-in into smartphones of LG Nexus4, Nokia Lumia 920/820, HTC Droid, etc., or those in the accessory-sleeves for Apple iPhone, Samsung S3/S4, etc. These wireless power receivers in smartphones and other portables will stop working if placed on top of higher power wireless power transmitters if such techniques as described herein were not implemented.